Conventional automotive electric power supply systems include: a generator that is interposed in a pathway that transmits movement driving forces of an automotive vehicle, and that generates electric power on passage of an excitation current to a field coil from a main electric power source of the vehicle; a capacitor that stores the electric power that is generated by the generator; a DC/DC converter that converts high voltage from the generator to a voltage of the main electric power source; and a power generation controlling means that operates the generator during deceleration of the automotive vehicle in response to operating states of the vehicle and a charged state of the capacitor. In conventional automotive electric power supply systems, deceleration energy is recovered as electric power during deceleration of the vehicle by operating the generator in response to the operating states of the vehicle and the charged state of the capacitor to generate a higher voltage than the main electric power source, and storing the generated electric power in the capacitor, and the generated high voltage is converted to the voltage of the main electric power source by the DC/DC converter and used (see Patent Literature 1, for example).